Jerome Wenger NanoBioPhotonics

The Green function plays a central role in wave propagation, as it describes the response of a system to an arbitrary impulse. However, measuring it experimentally is very challenging since it needs to be measured in both amplitude and phase with deep subwavelength spatial resolution. These highly demanding requirements have significantly limited the experimental attempts towards measuring the Green function in optics.

In a recent Phys Rev X publication, we describe a method to measure both the real and imaginary parts of the Green function by recording the mutual impedance between two dipoles at microwave frequencies. The effectiveness of this approach is demonstrated by the full characterisation of the complex Green function inside a resonant planar cavity of parallel or non-parallel mirrors at a ultrahigh resolution 100x below the wavelength. With this data, we are able to investigate various aspects of resonant dipole-dipole interaction and cooperative effects inside a photonic cavity.

Significance:

- We develop a new general methodology to fully measure the Green function in both amplitude and phase at ultrahigh spatial resolution.

- Our technique provides a powerful way to solve problems for which no analytic solution exists and where numerical simulations demand excessive computational resources.

- We characterize cooperative effects such as superradiance and cooperative Lamb shift inside a cavity which is relevant for various communities such as cavity QED, photovoltaics and nanophotonics.

Plasmonic nano-optical tweezers offers unprecedented abilities to manipulate nano-objects. However, measuring the trap stiffness at nanoscale dimensions remains a technical challenge, and as a consequence, very few reports have explored plasmonic designs to optimize their trapping performance.

In a recent publication released in Nanoscale, we detail a new approach to measure the trap stiffness taking advantage of the fluorescence emission of the trapped nanoparticle used for calibration. We relate our measurements to numerical simulations, and provide simple rules to optimize the design of the plasmonic structure to improve its trapping performance.

Significance:

- We optimize the design of double nanohole apertures used for plasmonic trapping. Compared to the previous state-of-the-art, our double nanohole structure achieves a 10x higher trap stiffness.

- We detail a general optical method to measure the trap stiffness in virtually any nano-optical tweezers experiment.

- We show that numerical simulations of the peak local intensity inside the structure is a simple and reliable measure to optimize the plasmonic design and the trap performance.

Also freely available on ArXiv.

An international study entitled "updated science-wide author databases of standardized citation indicators" recently measured the impact of scientific citations. 160,000 scientists -corresponding to the world's top 2% in their respective fields- have seen their output analyzed from 1960 to 2019 and classified into different scientific fields.

In this list, Jerome Wenger is ranked #708 out of 56325 researchers in optics (excluding self-citations). His citation score would correspond to a level #1402 out of 75210 researchers in nanosciences.

Our laboratory Institut Fresnel gathers a total of 16 of his researchers into this top-2% international list, which makes it likely the #1 optics lab in France in number of ranked PIs.

Let's make these number even better. Now that we have a measurement standard, the race is on.

Plasmonic nano-optical trapping has revolutionized the use of optical tweezers, and a lot of attention has been devoted to the optical gradient force. However, the metal absorbs part of the incident light, leading to a temperature gradient responsible for an additional thermophoretic force which remains largely overlooked in plasmonic trapping.

In a recent Nano Letters publication, we experimentally disentangle the thermophoretic force contribution from the optical gradient force in double-nanohole plasmonic trapping. Our approach uses different surfactants which allow to tune the thermophilic or thermophobic response of the nanoparticles and set the relative strength of the thermophoretic force. This uniquely demonstrates that the choice of the surfactant can play a determining role in the outcome of the plasmonic trapping experiment.

Significance:

- Our method to measure separately the thermal and the optical forces is very general and can be easily extended to other plasmonic designs.

- The nano-optical trap performance can be significantly improved by properly choosing the surfactant to take advantage of the thermophoretic force.

Also freely available on Arxiv 2011.10263

After a 3-year endeavour, Aleksandr Barulin defended his PhD on Dec 4th 2020.

Thesis committee:

- Thomas Ebbesen, ISIS, Université de Strasbourg, France, President

- Niek van Hulst, ICFO-Institut de Ciences Fotoniques, Spain, Reviewer

- Christian Eggeling, Friedrich-Schiller-Universität Jena, Germany, Reviewer

- Hervé Rigneault, Institut Fresnel, France, Examiner

- Aude Lereu, Institut Fresnel, France, Examiner

- Jérôme Wenger, Institut Fresnel, France, Supervisor

Read the thesis abstract here.